Cellulose Solubility in Deep Eutectic Solvents: Inspecting Quantitative Hydrogen-Bonding Analysis

According to the Kamlet-Abraham-Taft (KAT) polarity parameters (α, β, π*), polymers and solvents can be categorized as hydrogen-bond (H-bond) acidic (α>β) or H-bond basic (α<β). Recently, we proposed a quantitative hydrogen bonding (QHB) analysis to predict the solubility of polymers in ionic liquids (ILs) using the product of ΔαΔβ<0 as an indicator, where Δα is the difference between the H-bond acidic parameters of the polymer and IL, and Δβ is the difference in their basicity, while the prerequisite of the “complementary” principle (i.e., that one component is H-bond acidic and the other is basic) is satisfied. Here, the applicability of QHB analysis was first confirmed by testing the solubilities of carefully chosen polymer/deep eutectic solvent (DES) pairs, as the DESs were eutectic mixtures dominated by hydrogen bonding interactions. Then, our attention focused on the solubility of cellulose in DESs. Our testing results as well as the typical published results were summarized, which indicate that the potential DESs for cellulose dissolution and regeneration must be of the H-bond basic type because the “complementary” principle should be satisfied as a prerequisite. However, the H-bond basic DESs investigated in this study do not show the superior solubility of cellulose that has been commonly observed for H-bond basic ILs, even if the criterion of ΔαΔβ<0 is satisfied for both DESs and ILs. Possible reasons for this discrepancy are given to understand the varying effectiveness in cellulose dissolution for H-bond basic DESs and ILs.